Despite all those people not being able to identify the cause of the problem; I can suggest several potentially valuable things which were probably missed, that ought to be done. Unfortunately, when you present a second opinion to people who have already failed; their usual tendency is to dismiss the validity of the dissenting opinion. So, if you intend to follow these suggestions, it might be better to have them carried out by someone who has not been involved in the previous tests. These ten things should be done in the order in which they are listed. When you reach the point where the stalling stops, you don't need to proceed any further through the list (unless you really want to).
1> If the engine in your car has adjustable valve lash; the valve clearances should be checked and adjusted to manufacturer's specifications.
2> A compression test should be run on the engine. If ANY cylinder has a pressure reading which is more than 10% different from any other cylinder's pressure; the engine will not be able to be adjusted properly. In that case the cylinder head (or possibly the entire engine) will need to be reconditioned or replaced.
3> If the engine passes the compression test; the spark plug type, gap, and model should be checked against the catalog from that particular plug manufacturer; in order to make sure the correct spark plug and gap setting is being used in this motor. If any plugs appear worn or partly fouled, or there is ANY doubt about the suitability of the plugs; ALL the plugs should be replaced.
4> If this vehicle has the 3.2 liter V-6 engine; there is a VERY unusual caution which applies to that particular engine: The V-6 engine has coil on plug ignition; the coil boots used on cylinders 1 through 5 are all the same, but the coil boot for cylinder # 6 has a different part number (and is probably a different length or shape). If the number 6 coil boot has been installed on any other cylinder; or the # 6 coil boot is the same as all the others; then the coil boots should ALL be replaced with the proper parts. Similarly, there should be spark plug tube seals in each spark plug tube. If they are not there, or are damaged; they must be replaced.
5> The throttle position sensor should be replaced with an AC Delco sensor, part # 213894.
6> While the engine is idling at normal operating temperature; disconnect the vacuum hose from the EGR valve, and connect a vacuum gauge to that hose. If any vacuum is measured in the hose; or if the engine smooths out when the hose is disconnected from the EGR valve, the EGR vacuum regulator valve's electrical connector should be disconnected, and the vacuum in the hose checked again. If there is still vacuum in the hose; the EGR vacuum regulator valve should be replaced with an AC Delco vacuum regulator valve # 214565.
7> The EGR valve should be removed and inspected for deposits which may either plug the valve, or prevent it from closing completely. If it cannot be cleaned thoroughly, it should be replaced.
8> The manifold pressure (MAP) sensor should be checked for vacuum leaks anywhere in its vacuum supply hose. If no vacuum leaks are found, the MAP sensor should be replaced with a Delphi sensor; part # PS 10002.
9> The idle air control valve should be tested for proper operation; if it does not operate properly the electric activation circuit should be tested and repaired. If the activation circuit works properly, the IAC valve should be replaced.
10> The air cleaner charge temperature sensor should be replaced.
I hope this helps!!!